1. Field of the Invention
The present invention relates to composite ferrite compositions superior in high frequency characteristics and to electronic components to which said composite ferrite compositions are applied.
2. Description of the Related Art
In recent years, a frequency band used for such as portable phones, PC, etc. is made a high frequency, and there are already more than one standard having several GHz. Products to remove noise corresponding to signals of such high frequency is demanded. A multilayer chip coil is mentioned as major example thereof.
Electric characteristics of the multilayer chip coil can be evaluated by impedance. The impedance characteristic is greatly influenced by permeability of base body materials and frequency characteristic of base body materials until 100 MHz band. Further, impedance of the GHz band is influenced by stray capacitance between counter electrodes of the multilayer chip coil. Three following methods are mentioned to reduce stray capacitance between counter electrodes of the multilayer chip coil: extending distance between counter electrodes, reducing area of the counter electrode, and reducing dielectric constant between counter electrodes.
In the following Patent Article 1, terminals are formed on both ends of a magnetic flux direction generated by a coil energization, in order to reduce the stray capacitance. According to the invention of Patent Article 1, the distance between an internal electrode and a terminal electrode is extendable, and also an opposing area between the internal electrode and the terminal electrode can be reduced. Thus, frequency characteristic is expected to be extended to the high frequency.
However, according to Patent Article 1, the stray capacitance between internal electrodes is not reduced, and there is a room for further improvement on this matter. Further, extending distance between the internal electrodes and reducing area of the internal electrode are improving methods involving structural change of the multilayer chip coil, and are largely influential to the other characteristics and to size and forms of the multilayer chip coil. Extending distance between internal electrodes is influential to size of the products. Thus, it is difficult to apply to chip devices, which require miniaturization. In addition, reducing area of the internal electrode involves a problem of DC resistance increase.
Recently, Ni—Cu—Zn based ferrite is often used as the base body material of the multilayer chip coil. Ni—Cu—Zn based ferrite is a magnetic ceramic which can be fired at around 900° C., and thus Ni—Cu—Zn based ferrite is often used. Ni—Cu—Zn based ferrite can be fired at around 900° C., and that simultaneous firing with Ag used as the internal electrode is possible. Further, dielectric constant of Ni—Cu—Zn based ferrite is around 14 to 15, which is high. It is determined difficult to lower the dielectric constant of Ni—Cu—Zn based ferrite.
According to Patent Article 2 described below, composite material is manufactured by mixing Ni—Cu—Zn based ferrite and low dielectric constant nonmagnetic substance. Said composite material is applied as the base body material. Silica glass, borosilicate glass, steatite, alumina, forsterite, zircon are mentioned as said low dielectric constant nonmagnetic substance. According to the invention described in Patent Article 2, dielectric constant of the composite material, obtained by mixing Ni—Cu—Zn based ferrite and low dielectric constant nonmagnetic substance, is lowered relative to the dielectric constant of Ni—Cu—Zn based ferrite.
However, according to Patent Article 2, in case when a glass-based material, such as silica glass, borosilicate glass, and the like, is used as main component of the low dielectric constant nonmagnetic substance, permeability of the composite material remarkably declines. It is conceivable that this is due to the glass-based material causing grain growth inhibition or magnetic path separation of magnetic substance. In addition, Ni—Cu—Zn based ferrite and the glass-based material is highly reactive, and thus, forms a hetero phase. Therefore, it is most likely to cause short-circuit by simultaneous firing with Ag-based conductor; and it is inappropriate for an Ag-based conductor applied multilayer coil.
On the other hand, in case when ceramic materials which are not the glass-based material, such as steatite, alumina, forsterite, zircon, etc., is main component of the low dielectric constant nonmagnetic substance, the reaction between Ni—Cu—Zn based ferrite and the ceramic material is difficult to occur, and hardly forms the hetero phase. However, there is a problem of the sintering property in case when ceramic material is used as main component of the low dielectric constant nonmagnetic substance. It is conceived that sintering of the composite material is difficult at firing temperature of 900°, which is possible for co-firing with internal electrode Ag.
The invention according to Patent Article 3 shows an application of a foamed ferrite. Namely, in Patent Article 3, burn-out material is mixed with the magnetic ceramic, pores are manufactured after sintering, and resin or glass is impregnated to the pores. Low dielectric constant is achieved by using the pores. In addition, resin or glass is impregnated to the pores in order to cover demerit of the foamed ferrite, which strength is weakened. Further, there is no problem with characteristics and sintering properties of the invention according to Patent Article 3.
However, according to the invention of Patent Article 3, a terminal electrode cannot be directly formed on the foamed ferrite since the ferrite contains many pores. Therefore, the ferrite with fewer pores must be used at the part where the terminal electrode is formed; and there is a fault that structure becomes complicated. In addition, grain size of the foamed ferrite after firing tends to be small relative to the grain size of the ferrite with fewer pores. Therefore, moisture resistance and so on is most likely deteriorated in case when the foamed ferrite is used.
[Patent Article 1] Japanese Unexamined Patent Publication No. H11-026241
[Patent Article 2] Japanese Unexamined Patent Publication No. 2002-175916
[Patent Article 3] Japanese Unexamined Patent Publication No. 2004-297020